During processing of semiconductor wafers in a vertical furnace, a wafer boat accommodating a relatively large number of wafers in a horizontal orientation and a vertically spaced relationship is accommodated in the furnace. In a standard wafer boat the wafers are supported at three or more peripheral (wafer edge) positions. However, at high temperatures (e.g., >1000° C.) the yield strength of the wafers decreases and the wafers can sag under their own weight, they can deform as a result of thermally induced stresses or they can deform as a result of both effects. Wafers with large diameters are more susceptible to this effect than wafers with small diameters, as the thickness of the wafers does not increase proportionally with their diameter.
In U.S. Pat. No. 5,219,079 a wafer boat was proposed comprising susceptor plates to avoid warping of the wafers. The plates are each provided with a large recess to allow loading of a wafer onto each plate with a robot end effector, contacting the wafer at its lower surface. However, it was found that the relatively large recess resulted in local deformation of the wafer. Other boats with extended support area for the wafers are proposed in the art like ring type support boats. However, for high temperature processing of 300 mm wafers, the proposed supports are not adequate and still give rise to plastic deformation of the wafers.
In separate efforts, JP 63102225 suggests a wafer boat comprising susceptor plates that supports the wafers over their entire lower surfaces. This boat is used to keep reactive gas turbulence away from the wafer, thus improving the uniformity of a film deposited by CVD on each wafer. Wafers need to be loaded manually onto the susceptor plates. This is not acceptable in present-day processing, where the manual handling of 300 mm wafers is not a workable option.
It is therefore an object of the present invention to provide a method and device for heat treating 300 mm wafers at temperatures above 1000° C. wherein the wafers are handled automatically and the occurrence of plastic deformation of the wafers is minimized or eliminated.